With a resistive magnet type of nuclear magnetic resonance imaging apparatus, a static magnetic field is generated by passing a fixed drive current through a static magnetic field coil. If the resultant static magnetic field is stable, then the 2-dimensional reverse Fourier transform can be applied to the measured data for thereby correctly reconstructing a image of the body of an individual under diagnosis, with an image of good quality being obtained.
However even if the power source used to generate the static magnetic field is extremely stable, so that stable constant-current drive is applied, the static magnetic field can vary by several p.p.m due to changes in the shape of the static magnetic field coil as a result of variations in the ambient temperature. For this reason, artifacts will be produced in the image which is obtained, rendering it impossible to derive a satisfactory 2-dimensional Fourier image of the body which is under diagnosis.
Proposals have been made in the prior art for producing nuclear magnetic resonance imaging which will overcome this problem, specifically in Japanese Patent Provisional Publication No. 57-66346 and in Japanese Patent Provisional Publication No. 57-192541. In the case of Japanese Patent Provisional Publication No. 57-66346, a standard specimen is disposed within the static magnetic field, and the amount of variation of the static magnetic field is derived on the basis of an NMR signal which is detected from the standard specimen. The measured data which is obtained for the body which is under diagnosis is corrected, based upon the amount of variation of the static magnetic field that is obtained in this way. However with this method, it is necessary to employ a standard specimen, and a measurement unit for the standard specimen. In the case of Japanese Patent Provisional Publication No. 52-192541, an NMR signal is detected upon cessation of application of a gradient magnetic field to the body which is under diagnosis, and the amount of variation of the static magnetic field is obtained from this NMR signal, whereby the current that is passed through the static magnetic field correction coil is controlled such as to bring the amount of variation of the static magnetic field to zero. With this method it is not necessary to use a test specimen or a measurement unit for the test specimen. However due to the fact that it is necessary to halt application of the gradient magnetic field in order to measure the NMR signal produced by the body which is under diagnosis at the time of cessation of the gradient magnetic field, the measurement sequence must be different from the normal sequence, i.e. a special measurement sequence must be adopted.